1. Field of the Invention
The present invention relates to a lapping apparatus used in finishing, and polishing paper machine headbox components, and relates more particularly to lapping an apron floor or apron lip of a paper machine headbox.
2. Description of the Prior Art
Paper produced by paper machines is often subject to problems such as barring, streaking or ridging in its final form. Many of these problems can be traced to basis weight streaks caused by an uneven jet of paper stock coming out of a headbox. It has been determined that subtle variations in an apron floor in a headbox can have a significant negative impact on basis weight profile.
Conventionally, a headbox apron floor is finished in a sequence of planing or milling, grinding, mechanical polishing, electro-polishing and even lapping to produce a more uniform flat surface. Recently, lapping has gained acceptance as the preferred method to achieve desired flatness and finish, to prevent basis weight streaks or other related problems.
U.S. Pat. No. 5,567,273 and Canadian Patent No. 2,121,967 (both of which are hereby incorporated by reference herein) describe a method of reducing surface irregularities in paper machine headbox components in order to prevent streaking and other degradation of paper produced. In these prior art patents, certain lap tools are described which are used to accomplish the method of lapping a surface, namely, headbox components.
Prior art lap tools, like those described in the mentioned prior art patents, are generally solid circular plates which are operationally connected to a milling machine. The milling machine rotates the lap tool over the piece being lapped during the lapping operation. The lap tool and the piece being lapped generally move in a linear direction, one with respect to the other. On the working surface of the lap tool, namely, the surface which is driven over and on the piece being lapped, circular sand paper disks, commonly referred to as "daisies", are adhesively attached. These daisy sand paper disks are common articles of commerce and can be obtained from any number of sand paper suppliers, such as, for example, 3M or Buehler. The circular daisy sand paper disks are generally located in a circular pattern around the periphery of the lap tool. However, due to the speeds and pressures needed to obtain the required surface finish on the apron floor, it has been found that the prior art daisy sand paper disks do not hold up well under normal operating conditions. Many times, the daisy pads wear out too quickly and/or easily come off of the lap tool. Such results are unacceptable in that there is metal to metal contact between the lap tool and the piece being lapped. Because of the problems associated with using stick-on adhesive lapping sand paper pads, machine operators run the lap tool at slower than optimal speeds and pressures, thereby increasing machine cycle time, which results in excessive operating costs associated with lapping a particular piece.
Prior art lap tools, like those described in the previously mentioned patents, are driven by a standard milling machining tool driver. Such prior art drivers are ridged, one-piece constructions that are attached to the milling machine and the lapping tool. When driving a lapping tool over a surface, the milling machining exerts a pressure between the lapping tool and the piece being lapped. Because the tool driver is of a ridged, one-piece construction, if it is lowered down onto the piece being lapped too quickly or too much pressure is exerted, the apron floor or piece being lapped can be damaged by being gouged. Such damage is extremely detrimental to an apron floor and must be fixed to prevent the problems previously mentioned that can occur in the final paper product. Because of the high degree of flatness and surface finish requirements in apron floor surfaces, in order to prevent basis weight streaks, fixing such damage by smoothing out gouges is extremely costly and requires extensive machining time.
What is needed is a lapping tool which eliminates the heretofore mentioned problems. What is needed is a lapping tool which reduces machining cycle time by using a more aggressive grind and lapping material. Such a tool must be able to withstand higher operating speeds and pressure than heretofore obtained. Additionally, what is needed is a lapping tool which does not gouge a surface being lapped when lowered down onto the surface either too quickly or at too high a pressure.